2.2. Channels Dialog

Figure 15.7. The Channels dialog

The Channels dialog is the main interface to edit, modify and manage your
channels. Channels have a double usage. This is why the dialog is divided
into two parts: the first part for color channels and the second part for
selection masks.

Color channels apply to the image and not to a
specific layer. Basically, three primary colors are necessary to render
all the wide range of natural colors. As other digital software,
GIMP uses Red, Green, and Blue as primary colors. The
first and primary channels display the Red,
Green, and Blue values of each
pixel in your image. Next to the channel name is a thumbnail displaying a
grayscale representation of each channel, where white is 100% and black is
0% of the primary color. Alternatively, if your image is not a colored but
a Grayscale image, there is only one primary channel called
Gray. For an Indexed image with a fixed number of
known colors there is also only one primary channel called
Indexed. Then there is a optional channel called
Alpha. This channel displays transparency values of
each pixel in your image (See Alpha
Channel in Glossary). In front of this channel is a thumbnail
displaying a grayscale representation of the transparency where white is
opaque and visible, and black is transparent and invisible. If you create
your image without transparency then the Alpha channel is not present, but
you can add it from the Layers dialog
menu. Also, if you have more than one layer in your image,
GIMP automatically creates an Alpha channel.

Note

GIMP doesn't support CMYK or YUV color models.

Figure 15.8. Representation of an image with channels

Red channel

Green channel

Blue channel

Alpha channel

All channels

The right image is decomposed in three color channels (red, green, and
blue) and the Alpha channel for transparency. On the right image the
transparency is displayed as a gray checkerboard. In the color channel
white is always white because all the colors are present and black is
black. The red hat is visible in the red channel but quite invisible in
the other channels. This is the same for plain green and blue which are
visible only in their own channels and invisible in others.

2.2.1. Activating the Dialog

from the Tab menu in any dockable dialog by clicking on
and selecting
Add Tab → Channels.

In the Windows menu, there is a list of
detached windows which exists
only if at least one dialog remains open. In this case, you can raise
the “Channels” dialog from the image-menu:
Windows → Channels.

2.2.2. Using the Channel dialog

2.2.2.1. Overview

The top channels are the color channels and the optional Alpha
channel. They are always organized in the same order and they cannot
be erased. Selection masks are described below and displayed as a list
in the dialog. Every channel appears in the list with its attributes,
including a thumbnail and its name. A right-click in a channel list
entry opens the channel context
menu.

2.2.2.2. Channel attributes

Every channel is shown in the list with its own attributes, which are
very similar to the layer
attributes:

Channel visibility

By default every channel and thus every color value is visible.
This is indicated by an “open eye” icon. Clicking
on the eye-symbol (or the space if the channel is not visible)
will toggle the visibility of the channel.

Chain channels

The channels representing selection masks (the new channels in
the lower part of the channel list) may be grouped using the
button with the “chain” symbol. Then these channels
are all affected in the same way by operations applied to any
one of them.

Primary color channels (the default channels in the upper part
of the channel list) may be grouped too. By default, all color
channels (and the alpha channel) are selected, their list
entries are highlighted. Operations will be performed on all
channels. By clicking on a channel list entry you can
deactivate this channel. Operations like
colorizing a layer
will then be applied to the selected (“grouped”)
channels only. Clicking again on the list entry will activate
the channel.

Thumbnail

A small preview-icon represents the effect of the channel. On
a selection mask, this preview can be enlarged by holding
click down on it.

Channel name

The name of the channel, which must be unique within the image.
Double-clicking on the name of a selection mask channel will
allow you to edit it. The names of the primary channels (Red,
Green, Blue, Alpha) can not be changed.

Caution

Activated channels appear highlighted (generally) in blue in the
dialog. If you click on a channel in the list you toggle activation
of the corresponding channel. Disabling a color channel red, blue,
or green has severe consequences. For instance if you disable the
blue channel, all pixels from now on added to the image will not
have blue component, and so a white pixel will have the yellow
complementary color.

2.2.2.3. Managing channels

Under the channel list is a set of buttons allowing you to perform
some basic operations on channel list.

Edit Channel Attributes

Only available for selection masks. Here you can change the
Channel name. The other two parameters
affect channel visibility in the image window; they control
Opacity and color used for the mask in the
image window. A click on the color button displays the GIMP
color selector and then you can change the mask color.

New Channel

You can create here a new channel. The displayed dialog lets
you set Opacity and mask color used in
the image to represent the selection. (If you use the
New Channel button in Channel Menu,
you can create this new channel with the options previously
used by pressing the Shift key when clicking).
This new channel is a channel mask (a selection mask) applied
over the image. See
Selection Mask

Raise Channel

Only available for selection masks: you can here put the
channel up a level in the list. Press Shift
key to move channel to top of the list.

Lower Channel

You can here put the channel down a level in the list. Press the
Shift key to move the channel to bottom of the
list.

Duplicate Channel

You can create here a copy of the active channel. Name of new
channel is suffixed with a number.

Tip

You can also duplicate a color channel or the Alpha channel.
It's an easy way to keep a copy of them and to use them later
as a selection in an image.

Channel to Selection

Here you can transform the channel to become a selection. By
default the selection derived from a channel replaces any
previous active selection. It's possible to change this by
clicking on control keys.

Shift:
the selection derived from a channel is added to the
previous active selection. The final selection is merged
from both.

Ctrl:
the final selection is the subtraction of selection derived
from a channel from the previously active one.

Shift+Ctrl:
the final selection is the intersection of selection derived
from a channel with the previously active one. Only common
parts are kept.

Delete Channel

Only available for selection masks: you can here delete the
active channel.

2.2.2.4. Channels Context Menu

Figure 15.9. Channel Context Menu

Overview

You can get the channel context menu by right clicking on a channel
thumbnail. This menu gives the same operations on channels as those
available from dialog buttons. The only difference concerns
transformation to selection operations, each of them having its own
entry in the menu.

Selection derived from channel is added to previous active
selection. Final selection is merging of both.

Subtract from Selection

Final selection is subtraction of selection derived from a
channel from previous active selection.

Intersect with Selection

Final selection is intersection of selection derived from a
channel with the previous active selection. Only common parts
are kept.

2.2.3. Selection masks

Figure 15.10. A selection composed out of channels.

Channels can be used to save and restore your selections.
In the channel dialog you can see a thumbnail representing the selection.
Selection Masks are a graphical way to build selections into a gray level
channel where white pixels are selected and black pixels are not selected.
Therefore gray pixels are partially selected. You can think of them as
feathering the selection, a smooth transition between selected and not
selected. This is important to avoid the ugly pixelization effect when you
fill the selection or when you erase its content after isolating a subject
from background.

Creating Selection Masks

There are several ways to initialize a selection mask.

From the image window menu
Select → Save to Channel
if there is an active selection.

In the image window the bottom-left button creates a
Quick Mask; the content
will be initialized with the active selection.

From the channel dialog, when you click on the
New channel button or from the context
menu. When created, this Selection mask appears in the
Channel dialog, named “Selection maskcopy” with
a queuing number. You can change this by using the context
menu that you get by right-clicking on the channel.

2.2.3.1. Using Selection Masks

Once the channel is initialized, selected (highlighted in blue), visible
(eye-icon in the dialog), and displayed as you want (color and opacity
attributes), you can start to work with all the paint tools. The colors
used are important. If you paint with some color other than white, grey,
or black, the color Value (luminosity) will be used to define a gray
(medium, light, or dark). When your mask is painted, you can transform
it to a selection by clicking on the
button (Channel to
Selection) or from the
context menu.

You can work in selection masks not only with the paint tool but also
with other tools. For instance, you can use the selection tools to fill
areas uniformly with gradients or patterns. By adding many selection
masks in your list you can easily compose very complex selections. One
can say that a selection mask is to a selection as a layer is to an
image.

Caution

As long as a selection mask is activated you are working in the mask
and not in the image. To work in the image you have to deactivate all
selection masks. Don't forget also to stop displaying masks in the
image by removing the eye icon. Check also that all RGB and Alpha
channels are activated and displayed in the image.

2.2.4. Quick Mask

Figure 15.11. Dialog Quick Mask

A Quick Mask is a
Selection Mask
intended to be used temporarily to paint a selection. Temporarily means
that, unlike a normal selection mask, it will be deleted from the
channel list after its transformation to selection. The
selection tools
sometimes show their limits when they have to be used for doing complex
drawing selection, as progressive. In this case, using the QuickMask is a
good idea which can give very good results.

2.2.4.1. Activating the dialog

The QuickMask can be activated in different ways:

From the image menu:
Select → Toggle QuickMask.

By clicking the left-bottom button showed in red on the screenshot.

By using the
Shift+Q
shortcut.

2.2.4.2. Creating a Quick Mask

To initialize a Quick Mask,
click the bottom-left button in the image window. If a selection was
active in your image, then its content appears unchanged while the
border is covered with a translucent red color. If no selection was
active then all the image is covered with a translucent red color.
Another click on the bottom-left button will deactivate the quick mask.

From the channel dialog you can double click on the name or the
thumbnail to edit the QMask attributes. Then you
can change the Opacity and its filling color. At
every moment you can hide the mask by clicking on the eye icon
in front of the QMask.

The mask is coded in gray tones, so you must use white or gray to
decrease the area limited by the mask and black to increase it. The area
painted in light or dark gray will be transition areas for the selection
like feathering. When your mask is ready, click again on the bottom-left
button in the image window and the quick mask will be removed from the
channel list and converted to a selection.

Quick mask's purpose is to paint a selection and its transitions with
the paint tools without worrying about managing selection masks. It's a
good way to isolate a subject in a picture because once the selection is
made you only have to remove its content (or inverse if the subject is
in the selection).

2.2.4.3. Using Quick Mask with a gradient

Description

Screenshot of the image window with activated QuickMask. As long as
the Quickmask is activated, all operations are done on it. A
gradient from black (left) to white (right) has been applied to the
mask.

The QuickMask is now disabled. The selection occupies the right half
part of the image (marching ants) because the limit of the selection
is at the middle of the gradient.

A stroke is now added during the enabled selection. Weird! The
gradient, although not visible, remains active all over the image,
in selected and non selected areas!

After the QuickMask Button is pressed, the command generates a
temporary 8-bit (0-255) channel, on which the progressive selection
work is stored. If a selection is already present the mask is
initialized with the content of the selection. Once QuickMask has been
activated, the image is covered by a red semi-transparent veil. This
one represents the non-selected pixels. Any
paint tool
can be used to create the selection on the QuickMask. They should use
only grayscale color, conforming the channel properties, white enabling
to define the future selected place. The selection will be displayed as
soon as the QuickMask will be toggled but its temporary channel will
not be available anymore.

Tip

To save in a channel the selection done with the Quickmask select in
the image menu
Select/Save to Channel